1. Field of the Invention
The present invention relates to methods for the preparation of a rigid polyurethane foam, and more particularly, to methods for the preparation of a rigid polyurethane foam having improved heat insulating properties by using an emulsified nucleating agent.
2. Description of the Related Art
To obtain rigid polyurethane foam, CFCs (chlorofluorocarbons) such as trichlorofluoromethane or dichlorofluoromethane, are conventionally used as a blowing agent. However, CFCs are regarded as environment-destroying materials, because they destroy the ozone layer or cause the green house effect. Thus, the production and use of CFCs have recently been prohibited, and are being replaced by other substituents.
As substituents for CFCs, HCFCs (hydrochlorofluorocarbons) have been used as a blowing agent. HCFCs are less destructive of the ozone layer than CFCs. The closed cell size of the rigid polyurethane foam formed using HCFCs is 200-300 μm, which is greater than a closed cell size of the rigid polyurethane foam formed using CFCs. But the rigid polyurethane foam using HCFCs has excellent heat insulating properties (adiabatic index: 0.0144 mW/mk) due to the low gas adiabatic index of the HCFCs (adiabatic index: 0.0094 mW/mk). However, since HCFCs also may destroy the ozone layer to a certain degree, their use is being gradually reduced.
Meanwhile, a hydrocarbon blowing agent such as cyclopentane, which does not destroy the ozone layer, was recently proposed.
However, since the rigid polyurethane foam formed using cyclopentane has a large closed cell size of 200-300 μm, and the adiabatic index of cyclopentane is 0.012 mW/mk, which is higher than the adiabatic index of CFCs and HCFCs, the heat insulating properties of the rigid polyurethane foam are ineffective (adiabatic index: 0.0165 mW/mk). Accordingly, a larger volume is needed for rigid polyurethane foam formed using cyclopentane to obtain the same adiabatic effect as the adiabatic effect obtained by forming the rigid polyurethane foam formed using CFCs and HCFCs.
To solve the above and/or other problems, a nucleating agent, such as a perfluorinated alkene, has been used in the reaction. However, the resultant rigid polyurethane foam obtained shows poor heat insulating properties since the nucleating agent makes the reaction system unstable.